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I've had mine over 11,000 ft in the mountains of Colorado and it did very well. Obviously it a bit down on power at that elevation, but it did better than I expected it to. It held the speed limit all the way without having keep the pedal on the floor. The automatic grade braking was interesting, but I prefer to use manual mode going down hill.

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I just got back from Colorado trip which included a drive up Pikes Peak and other areas up around 14k feet. Handled it like a champ no issues what's so ever. This is with the 6.2, but I assume 5.3 would be no different. Would have liked to had 93 octane all I could find was 91. Seems west of the Mississippi 93 is not as readily available as in IL and places farther to the east .

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I just got back from Colorado trip which included a drive up Pikes Peak and other areas up around 14k feet. Handled it like a champ no issues what's so ever. This is with the 6.2, but I assume 5.3 would be no different. Would have liked to had 93 octane all I could find was 91. Seems west of the Mississippi 93 is not as readily available as in IL and places farther to the east .

89 was likely the best you could find in CO. At least without searching a ton.

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Weather has a lot to do with how much you loose, but here in Denver correction factors at the dyno range from about 1.25 down to 1.18. At 10000 ft, it's more like a 40-50% loss.

Honestly, at least with this truck, it doesn't seem that bad. It may just be a real world example of just how invasive the torque management is at lower elevations. You get up high and the computer progressively unleashes more of the potential. I would assume a truck with a good tune would "feel" the elevation more.

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Not a 1500, but my 2015 2500 6.0 just did a road trip to Wyoming. I did a E85 only trip this time. Been using E85 for the last year continuously. The 6.0 in the 2500 did a fantastic job at altitude. I think that could be partially due to the higher oxygen content of E85.

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The potential loss at altitude is based on "density altitude". This is primarily a calculation of barometric pressure and temperature. While humidity plays a role, it is much less influential. But, just so you know, high humidity means less oxygen. As altitude increases, the pressure decreases and therefore the amount of oxygen in a cubic foot of air decreases. As temperature increases, the amount of oxygen in a cubic foot of air decreases. As humidity increases, the amount of oxygen in a cubic foot of air decreases.

There's a significant difference between 5000 ft at 40F vs. 5000 ft at 80F. A "density altitude" value is indicative of the performance you can expect at that pressure, altitude, temperature, and humidity. So, run this little test. Plug in 5000 ft, 40F, 29.92 (altimeter setting), 20F dewpoint. Read the "density altitude" (=4979 ft). Then change just the temperature to 80F (=7499 ft). This means your engine will loose power on the 80F day vs the 40F day, like it went up another 2520 ft in altitude.

For an engine that adjusts fuel/air mixture as altitude increases, the loss is minimized (not minimal). Old carburetor engines lost a lot more than 3% per 1000 ft. Our EFI engines hang in there about the 3% loss/1000 ft.